Room Temperature Single Phase Li Insertion/Extraction Material for Use in Li-Based Battery

ABSTRACT

The invention relates to active materials for the manufacture of Li-based batteries. A crystalline nanometric powdered material with formula Li x (M, M′)PO 4 , in particular Li x FePO 4  (0≦x≦1), is disclosed, exhibiting single phase Li insertion/extraction mechanism at room temperature when used as positive electrode material in Li-based batteries. Compared to current LiFePO 4 , the novel material results in smooth, sloping charge/discharge voltage curves, greatly simplifying the monitoring of the state of charge of the batteries. The coexistence of mixed valence states for Fe (i.e. Fe III VFe II ) is believed to increase the electronic conductivity in the room temperature single phase Li x FePO 4  material, compared to state of the art two-phase materials. This, together with the nanometric size of the particles and their sharp monomodal size distribution, contributes to the exceptional high-rate capability demonstrated in batteries.

This application is a national stage application of InternationalApplication No. PCT/EP2008/002195, filed Mar. 19, 2008, which claimspriority to European Patent Application No. 07290328.9, filed Mar. 19,2007, and U.S. Provisional Patent Application No. 60/907,732, filed Apr.16, 2007, the entire contents of which is hereby incorporated herein byreference.

This invention relates to crystalline nanometric materials, inparticular to Li_(x)FePO₄ (0≦x≦1) powder, showing an unusual singlephase Li insertion/extraction mechanism at room temperature (25° C.)when used as positive electrode material in Li-based batteries.

Some years after the original work from Padhi et al. (J. Electrochem.Soc., 144, 1188 (1997)) was published, phospho-olivines LiMPO₄ (with Mis Fe, Mn, Co . . . ) now appear to be potential candidates as positiveelectrode materials for rechargeable lithium batteries. Thanks to smartprocessing, e.g. by carbon coating, Li⁺ ions may be extracted out ofLiFePO₄ leading to room-temperature capacities of about 160 mAh/g, i.e.close to the theoretical capacity of 170 mAh/g. The room-temperature Liinsertion/extraction is well known, e.g. from WO2004/001881, to proceedat 3.45 V V5. Li⁺/Li, in a two-phase reaction between LiFePO₄ and FePO₄.Note that, as raised by Striebel et al. (J. Electrochem. Soc., 152, A664(2005)) while making a compilation of tests of various carbon-coatedLiFePO₄ compounds, even if the matrix conductivity has been improved bycoating, the battery developer would welcome so-far inexistent compoundshaving a primary particle size in the 50-100 nm range and, overall,attempts should be made to minimise the particle size distribution, inorder to yield better power efficiency.

WO2004/056702 and WO2007/00251 teach techniques to decrease the averageparticle size down to the 140-150 nm range. Nevertheless, it is admittedby the skilled persons that decreasing the particle size below thesevalues would allow a further increase of the high-power performances.

Various authors, e.g. Yamada et al. (Electrochem. Solid State Let., 8,A409 (2005)) and in US2007/0031732, have shown that reducing theparticle size would allow some deviation from the well describedtwo-phase Li insertion/extraction behaviour. Indeed, materials showingsmall particle sizes exhibit some limited solid solution (i.e. singlephase) domains at room temperature, namely of Li-poor Li_(x)FePO₄(x<0.15) and of Li-rich Li_(y)FePO₄ (y>0.85). Although it was recognizedthat the x and y limits, which represent the boundaries of the two-phasedomain, may depend upon both the particle size and the particularconditions of the synthesis, materials with a significantly broadersingle phase domain were never obtained.

The recent discovery of a complete single phase Li_(x)FePO₄ (0≦x≦1)solid solution at temperatures of around 350° C. has spurred greatinterest in assessing its role in the performance of LiFePO4 as cathodematerial for Li-ion batteries. Nevertheless, whatever the x values, itwas clearly demonstrated that the solid solution could not be stabilisedat room temperature, thus making it of limited practical interest asstandard battery material (Delacourt et al., Nature Mat., 4, 254 (2005);Dodd et al., Electrochem. Solid State Let., 9, A151 (2006)).

The most obvious distinction between a single phase and a two-phaseinsertion/extraction mechanism is that the equilibrium potential (EMF)of a single phase system is composition-dependent, while that of atwo-phase system is constant over the entire composition range. A singlephase electrode will thus show a sloping voltage curve during charge ordischarge cycles: this is welcomed by the battery manufacturer as itenables monitoring of the state of charge at reduced cost compared tosystems presenting a flat voltage curve.

Also, it is now admitted that in the two-phase system LiFePO₄/FePO₄,both end members present very limited electronic conductivity, and thatno mixed valence state is present either in FePO₄ (Fe^((III))) or inLiFePO₄ (Fe^((II))) (Delacourt et al., Electrochem. Soc., 152, A913(2005). As emphasized by Chiang et al. in US2007/0031732, greaterpopulation of both Fe species at every point within the deintercalationrange could provide a higher electronic conductivity for the material. Agood material conductivity is particularly advantageous with respect tohigh drain applications.

The same quest for enhanced material conductivity applies to similaractive materials for Li batteries, such as LiMnPO₄ and in Li(Fe,M)PO₄(with M is Co and/or Mn), as reported respectively in the patentapplications WO2008-77447 and WO2008-77448.

Further, in US2006/0035150 A1, for the preparation of coated LiFePO₄,sources of Li, Fe and phosphate are dissolved in an aqueous solutiontogether with a polycarboxylic acid and a polyhydric alcohol. Upon waterevaporation, polyesterification occurs while a mixed precipitate isformed containing Li, Fe and phosphate. The resin-encapsulated mixtureis then heat treated at 700° C. in a reducing atmosphere.

In WO2007/000251 A1, a direct precipitation process is described forpreparing crystalline LiFePO₄ powder, comprising the steps of:

-   -   providing a water-based mixture having at a pH between 6 and 10,        containing a water-miscible boiling point elevation additive,        and Li^((I)), Fe^((II)) and p^((V)) as precursor components;    -   heating said water-based mixture to a temperature less than or        equal to its boiling point at atmospheric pressure, thereby        precipitating crystalline LiFePO₄ powder.

An extremely fine 50 to 200 nm particle size is obtained, with a narrowdistribution.

In US2004/0175614 A1, a process is disclosed for the manufacture ofLiFePO₄, comprising the steps of

-   -   providing an equimolar aqueous solution of Li¹⁺, Fe³⁺ and PO₄        ³⁻,    -   evaporating the water from the solution, thereby producing a        solid mixture,    -   decomposing the solid mixture at a temperature below 500° C. to        form a pure homogeneous Li and Fe phosphate precursor, and    -   annealing the precursor at a temperature of less than 800° C. in        a reducing atmosphere, thereby forming a LiFePO₄ powder.

The obtained powders have a particle size of less than 1 μm.

In Delacourt et al., Solid State Ionics 173 (2004) 113-118, thethermodynamics and kinetics governing the precipitation of pure powdersof phosphates phases for Li batteries are described. Optimizedelectrodes were synthesized through a chemical conductive carbon coatingat the surface of LiFePO₄ prepared by evaporation of anFe^(III)-containing aqueous solution.

The disclosed process is aimed at providing a material with a higherconductivity than that of conventional materials, and at solving themonitoring problem of the state of charge.

To this end, a powdered Li insertion/extraction material is disclosed,comprising Li_(x)(M,M′)PO₄ as an active component, wherein 0≦x≦1, M isone or more cations selected from the group consisting of Mn, Fe, Co,Ni, Cu, and M′ is an optional substitutional cation selected from thegroup consisting of Na, Mg, Ca, Ti, Zr, V, Nb, Cr, Zn, B, AI, Ga, Ge,Sn, characterized in that said material is a single phase material thatis thermodynamically stable at 25° C. during Li insertion/extraction,for x varying from less than 0.2 to more than 0.8. In the above formula,M is preferably Fe; moreover, an M to M′ molar ratio of more than 5, andpreferably of more than 8, is advised. When M is Fe and M/M′>5, then theinvented material is typically characterized by a crystallographic cellvolume lower than 291 Å³, preferably equal to or lower than 290 Å³, andmore preferably equal to or lower than 289 Å³. This volume is deducedfrom XRD measurements using a Pmna or Pmnb space group.

The invented material is a powder with a preferred particle sizedistribution with a d50 of less than 50 nm, and preferably between 10and 50 nm. A d99 of less than 300 nm, and preferably of less than 200 nmis advised. Moreover, a mono-modal particle size distribution where theratio (d90−d10)/d50 is less than 1.5, preferably less than 1.2, isadvised.

Another aspect of the invention concerns a process for the synthesis ofthe above-described Li_(x)(M,M′)PO₄ materials. The process comprises thesteps of:

-   -   providing a first water-based mixture having a pH between 6 and        10, containing a bipolar (i.e. water miscible) aprotic additive,        and Li and P precursors introduced as Li^((I)) and p^((V));    -   adding an M precursor as M^((II)), and an M′ precursor, to said        first waterbased mixture, thereby obtaining a second water-based        mixture;    -   heating said second water-based mixture to a temperature of less        than or equal to its boiling point at atmospheric pressure,        thereby precipitating the powdered Li insertion/extraction        material.

In a preferred embodiment, Li^((I)) is introduced as LiOH.H₂O, andp^((V)) as H₃PO₄. It is advisable to adjust the pH of the first mixtureby using a proper ratio of LiOH.H₂O and H₃PO₄. The also process coversthe synthesis Li_(x)(M,M′)PO₄, wherein M=Fe, M′ being absent, andwherein the pH of the first water-based mixture is between 6.5 and 8,and preferably between 6.5 and 7.5.

The bipolar aprotic additive is preferably selected and dosed so as toelevate the atmospheric boiling point of the second water-based mixtureto between 100 and 150° C., preferably between 105 and 120° C.

Dimethylsulfoxide is a preferred additive. The first water-based mixturecontains between 5 and 50% mol, and preferably between 10 and 30% mol ofdimethylsulfoxide.

In a still preferred embodiment, the precipitating powdered Liinsertion/extraction material is subjected to a thermal post-treatmentby heating it in non-oxidising conditions, at a temperature of up to650° C., and preferably of at least 300° C.

In a still preferred embodiment, an electron conducting substance, orits precursor, is added to either one or more of the first water-basedmixture, the second water-based mixture, and the powder before thethermal posttreatment.

The electronic conducting substance can advantageously be carbon, inparticular conductive carbon or carbon fibres, and the precursor of theelectron conducting substance can be a carbon-based polymerizablestructure.

Another aspect of the invention concerns a secondary Li-based battery,comprising an anode, an electrolyte and a cathode, said cathodecomprising the above-described material.

Yet another aspect of the invention concerns an electrode mix forsecondary Li-based batteries, comprising the above-described material.

A first embodiment is related to an electrode mix for secondary Li-basedbatteries with non-aqueous liquid electrolyte, comprising at least 80%wt of the invented material, characterised by a reversible capacity ofat least 75% of the theoretical capacity (about 170 mAh/g), when used asan active component in a cathode cycled between 2.5 and 4.5 V vs. Li⁺/Liat a discharge rate of 0.1 C at 25° C. The amount of additives (binderand carbon) in the electrode mixture can be limited to less than 20% wt,preferably to less than 10% wt, because the mixture, being pasted on acurrent collector, needs not to be self-supporting for this type ofbatteries.

A second embodiment is related to an electrode mix for secondaryLi-based batteries with non-aqueous gel-like polymer electrolyte,comprising at least 80% wt of the invented material, characterised by areversible capacity of at least 75% of the theoretical capacity whenused as an active component in a cathode cycled between 2.5 and 4.5 Vvs. Li⁺/Li at a discharge rate of 0.1 C at 25° C. The amount ofadditives in the electrode mixture can be as high as 20% wt in thiscase, because the mixture, being rolled in the form of a sheet to belaminated to a current collector, needs to be self-supporting duringassembly of this type of batteries.

A third embodiment is related to an electrode mix for secondary Li-basedbatteries with non-aqueous dry polymer electrolyte, comprising at least70% wt of the invented material, characterised by a reversible capacityof at least 75% of the theoretical capacity, when used as an activecomponent in a cathode cycled between 2.5 and 4.5 V vs. Li⁺/Li at adischarge rate of 0.1 C at 25° C.

A further embodiment concerns a secondary Li-based battery with anelectrode comprising nanometric powdered Li_(x)(M,M′)PO₄ as an activecomponent, wherein 0≦x≦1, M is one or more cations selected from thegroup consisting of Mn, Fe, Co, Ni, Cu, and M′ is an optionalsubstitutional cation selected from the group consisting of Na, Mg, Ca,Ti, Zr, V, Nb, Cr, Zn, B, AI, Ga, Ge, Sn, characterized in that thecontribution of said electrode to the EMF of the battery at 25° C.varies continuously with the state of charge by more than 0.05 V, for xvarying from 0.2 to 0.8. In the above formula, M is preferably Fe;moreover, an M to M′ molar ratio of more than 5, and preferably of morethan 8, is advised. When M is Fe and M/M′>5, then the said nanometricpowdered active component is typically characterized by acrystallographic cell volume lower than 291 Å³, preferably equal to orlower than 290 Å³, and more preferably equal to or lower than 289 Å³.This volume is deduced from XRD measurements using a Pnma or Pmnb spacegroup.

By “continuously varying EMF” is meant a continuously slopingcharge/discharge voltage curve. This slope, according to the presentinvention, amounts to at least 5 mV per inserted/extracted Li, andpreferably to at least 15 mV per inserted/extracted Li, and this alongthe complete charge/discharge cycle.

The said nanometric powdered active component has a preferred particlesize distribution with a d50 of less than 50 nm, and preferably between10 and 50 nm. A d99 of less than 300 nm, and preferably of less than 200nm is advised. Moreover, a mono-modal particle size distribution wherethe ratio (d90−d10)/d50 is less than 1.5, preferably less than 1.2 isadvised.

It should be noted that, in the invented material, M and M′ areconsidered as at least partially interchangeable, whilst howeverrespecting electroneutrality rules assuming Li^((I)), M^((II)),M′^((I) to (V)), and p^((V)).

Products finer than 10 nm are not particularly advisable, as they couldlead to processability problems during electrode manufacturing.

According to the invented process, the first water-based mixture has apH between 6 and 10, preferably 6 to 8, in order to avoid precipitationof Li₃PO⁴ as impurities.

Use is made of a bipolar additive as a co-solvent that will increase theprecipitate nucleation kinetics, and thus reducing the size of the roomtemperature single phase Li insertion/extraction Li_(x)FePO₄ (0≦x≦1)nanometric particles. In addition to being bipolar, i.e. miscible withwater, useful co-solvents should be aprotic, i.e. show only a minor orcomplete absence of dissociation accompanied by release of hydrogenions. Cosolvents showing complexation or chelating properties such asethylene glycol do not appear suitable as they will reduce the kineticsof precipitation of Li_(x)MPO₄ and thus lead to larger particle sizes.Suitable dipolar aprotic solvents are dioxane, tetrahydrofuran,N-(C₁-C₁₈-alkyl) pyrrolidone, ethylene glycol dimethyl ether,C₁-C₄-alkylesters of aliphatic C₁-C₆-carboxylic acids, C₁-C₆-dialkylethers, N,N-di-(C₁-C₄-alkyl)amides of aliphatic C₁-C₄-carboxylic acids,sulfolane, 1,3-di-(C₁-C₈alkyl)-2-imidazolidinone,N-(C₁-C₈-alkyl)caprolactam, N,N,N′,N′-tetra-(C₁-C₈-alkyl)urea,1,3-di-(C₁-C₈-alkyl)-3,4,5,6-tetrahydro-2(1H)-pyrimidone,N,N,N′,N′-tetra-(C₁-C₈-alkyl)sulfamide, 4-formylmorpholine,1formylpiperidine or 1-formylpyrrolidine, in particularN-(C₁-C₁₈-alkyl)pyrrolidone, N,N-di-(C₁-C₄-alkyl)amid-e of aliphaticC₁-C₄-carboxylic acids, 4-formylmorpholine, 1-formylpiperidine or1-formylpyrrolidine, preferably N-methyJpyrrolidone (NMP),N-octylpyrrolidone, Ndodecylpyrrolidone, N,N-dimethylformamide,N,N-dimethylacetamide, 4-formylmorpholine, 1-formylpiperidine or1-formylpyrrolidine, particularly preferably N-methylpyrrolidone,N,N-dimethylformamide, N,Ndimethylacetamide or hexamethylphosphoramide.Other alternatives such as tetraalkyl ureas are also possible. Mixturesof the abovementioned dipolar aprotic solvents may also be used. In apreferred embodiment, dimethylsulfoxide (DM50) is used as solvent.

It cannot be excluded that the novel room temperature single phaseinsertion/extraction material could lead to a two-phase system attemperatures well below 25° C., such as below 10° C. This phasetransition should however be reversible. Its effect should thereforeonly minimally affect the operation of batteries in most practicalcircumstances.

The disclosed process leads to an initial material that may containtraces of Fe(III). Due to the nanometric particle size, some Fe^((III))could arise from a deviation from stoichiometry at the surface of thematerial. The presence of Fe^((III)) could also be due to a secondamorphous phase, most likely LiFePO₄(OH) or FePO₄.nH₂O, at the surfaceof the crystals or at grain boundaries. The skilled person may minimizethe Fe^((III)) by working under reducing atmosphere or by relying onreducing agents such as hydrazine or SO₂. The possible Li deficit in theinitial material could moreover be compensated during the first fulldischarge cycle of the battery if the environment is able to provide thenecessary Li (as it is likely the case in many practical batteries).

Compared to state of the art LiFePO₄ materials, the advantages of theinvented material are:

-   -   a sloping charge/discharge curve, allowing direct monitoring of        the state of charge by simple potential measurement;    -   a nanometric particles size, which alleviates kinetic        limitations due to Li ion transport within the particles, and        allows fast charge/discharge of the battery;    -   a narrow particle size distribution, ensuring a homogeneous        current distribution within the battery; this is again        especially important at high charge/discharge rates, where finer        particles would get more depleted than coarser ones, a        phenomenon leading to the eventual deterioration of the        particles and to the fading of the battery capacity upon use; a        narrow particle size distribution furthermore facilitates the        manufacture of electrodes;    -   the coexistence of mixed valence state for Fe (i.e.        Fe^((III))/Fe^((II))), which is believed to increase the        electronic conductivity of the room temperature single phase        Li_(x)FePO₄ material compared to state of the art two-phase        materials, represented as (1−x)FePO₄+xLiFePO₄ (0≦x≦1).

SUMMARY OF THE FIGURES

FIG. 1: Galvanostatic charge/discharge curve of the invented material at25° C. and C/20 rate, showing a sloping voltage curve. The plot showsthe voltage of the battery as a function of the normalized capacity; 0%state of charge (SOC) corresponds to starting LiFePO₄ material, while100% corresponds to charged delithiated FePO₄ material.

FIG. 2: FEG-SEM picture of product of the Example, showing the smallparticle size and the sharp particle size distribution.

FIG. 3: Volumetric particle size distribution and cumulativedistribution (% vs. nm) for the product of the Example showing d50values about 45 nm, while the relative span, defined as (d90−d10)/d50,is about 1.2 (d10=25 nm, d90=79 nm).

FIG. 4: In situ XRD recorded at different states of charge and at 25° C.for invented material; 0% state of charge (SOC) corresponds to startingLiFePO₄ material, while 100% corresponds to charged delithiated FePO₄material.

FIG. 5: Evolution with composition of the cell parameters for LixFePO₄(0≦x≦1) calculated from the in situ XRD recorded at measured at 25° C.at different states of charge. This clearly shows a continuous solidsolution between LiFePO₄ and FePO₄ with continuous variation between thelimiting values of the cell parameters as the lithium concentrationvaries from 1 to 0.

FIG. 6: Galvanostatic charge/discharge curve of state of the artmaterial at 25° C. and C/20 rate, showing the constant voltage curve.The plot shows the voltage of the battery as a function of thenormalized capacity; 0% state of charge (SOC) corresponds to startingLiFePO₄ material, while 100% corresponds to charged delithiated FePO₄material.

FIG. 7: in situ XRD recorded at different states of charge for state ofthe art products; 0% state of charge (SOC) corresponds to startingLiFePO₄ material, while 100% corresponds to charged delithiated FePO₄material.

FIG. 8: Evolution with composition of the cell parameters for(1−x)FePO₄+xLiFePO₄ (0≦x≦1) calculated from the in situ XRD recorded atdifferent states of charge. This clearly shows a classical two-phasesystem, the proportion of each end member varying with the lithiumconcentration in the material.

EXAMPLE

The invention is further illustrated in the following example. In afirst step, DMSO is added to a solution of 0.1 M H₃PO₄, diluted in H₂Ounder stirring. The amount of DMSO is adjusted in order to reach aglobal composition of 50% vol. water and 50% vol. DMSO.

In a second step, an aqueous solution of 0.3M LioH.H₂O is added at 25°C. in a quantity so as to increase the pH up to a value between 6.5 and7.5, and leading to the precipitation of Li₃PO₄.

In a third step, a solution of 0.1M Fe(II) in FeSO₄.7H₂O is added at 25°C. This is believed to lead to the re-dissolution of Li₃PO₄. The finalLi:Fe:P ratio in the solution is close to 3:1:1. By adding the Fe(II)precursor after the pH of the solution has been set at a certain valuebetween 6.5 and 7.5, it is possible to perform a controlledprecipitation of Fe-species resulting in much lower particle sizes thanobtained in the prior art.

In a fourth step, the temperature of the solution is increased up to thesolvent boiling point, which is 108 to 110° C. After 6 h, the obtainedprecipitate is filtered and washed thoroughly with water.

The powdery precipitate is pure crystalline LiFePO₄, according to XRDmeasurements. The full pattern matching refinement done on XRD pattern(Pmnb space group) leads to cell parameters a=10.294 Å³, b=5.964 Å³ andc=4.703 Å³, corresponding to a crystallographic cell volume of 288.7 Å³.The FEG-SEM picture on FIG. 2 shows monodisperse small crystallineparticles in the 30 to 60 nm range. The volumetric particle sizedistribution of the product was measured by using image analysis. Asshown in FIG. 3, the d50 values is about 45 nm, while the relative span,defined as (d90d10)/d50, is about 1.2 (d10=25 nm, d90=79 nm).

A slurry is prepared by mixing the LiFePO₄ powder obtained with theprocess described above with 10% wt carbon black and 10% wt PVDF intoN-Methyl Pyrrolidone (NMP) and deposited on an AI foil as currentcollector. The obtained electrode containing 80% wt active material isused to manufacture coin cells, using a loading of 6 mg/cm² activematerial. The negative electrodes are made of metallic Li. The coincells are cycled in LiBF₄ based electrolyte between 2.5 and 4.0 V. FIG.1 shows that high reversible capacity is obtained at low rate with asloping voltage curve upon cycling characteristic of a single phase Liinsertion/extraction mechanism. It should be emphasised that the curveof FIG. 1 has been recorded in galvanostatic conditions, and, as such,only approximates the EMF of the electrode. The EMF is in this casevarying continuously as a function of Li insertion/extraction; the slopeof the EMF curve is thus definitely nonzero, although it might beslightly less pronounced than in the figure.

FIG. 4 shows in situ XRD data collected in the battery upon cycling. Itis clearly visible on FIG. 5 that the insertion/extraction proceeds fromLiFePO₄ to FePO₄ with a continuous evolution of the cell parameters,which evidences the presence of a single Li_(x)FePO₄ (0≦x≦1) phase. Italso emphasizes the good reversibility of this single phase mechanismupon cycling.

Counter Example

As a counter example, materials are synthesised according to the exampleillustrating WO2007/000251. Compared to the example according to theinvention, one notes that the order of addition of the reactants isdifferent; this change is of crucial importance with respect to thefinal particle size of the precipitated material, this latter beingabout 130 to 150 nm for product precipitated according to the mentionedprior art. It is understood that the difference between the presentinvention and this prior art lies in the fact that the Fe-precursor isadded to a solution having already a fixed and stable pH, between 6.5and 7.5; whilst in the prior art the Fe-precursor is added to a solutionhaving a pH of less than 6, whereafter the addition of the Li-precursorraised the pH to around 7. The Fe-precursor can also be added in a solidform.

Also, the obtained counter example material, as characterized byRietveld refinement from XRD pattern, shows a crystallographic cellvolume of 291.7 Å³.

With his material, batteries are prepared as described above. FIG. 6shows the charge/discharge curve of the prior art material at roomtemperature and C/20 cycling rate. There is a constant voltage plateau,which is characteristic of a two-phase Li insertion/extractionmechanism. It should be emphasised that the curve of FIG. 6 has beenrecorded in galvanostatic conditions, and, as such, only approximatesthe EMF of the cell. The EMF is in this case constant, the slope of theEMF as a function of Li insertion/extraction being essentially zero.

FIG. 7 shows in situ XRD recorded at different state ofcharge/discharge. The evolution of the cell parameters is illustrated inFIG. 8. This clearly shows a classical two-phase system, the proportionof each end member FePO₄ and LiFePO₄ varying with the lithiumconcentration in the material, as opposed to the product according tothe invention.

1. A Li insertion/extraction powdered material comprising particles of acrystalline material, the particles having a size distribution d50 ofless than 50 nm, wherein during Li extraction (discharging) from saidmaterial and Li insertion (charging) in said material, the Liinsertion/extraction powdered material remains a single phase materialthat is thermodynamically stable at 25° C. for any amount of Li presentin said material.
 2. The material of claim 1, wherein d50 is between 10and 50 nm.
 3. The material of claim 1, having a d99 of less than 300 nm.4. The material of claim 1, having a ratio (d90−d10)/d50 of less than1.5.
 5. A secondary Li-based battery with an electrode comprising the Liinsertion/extraction powdered material of claim 1 as an active componentwherein during discharging and charging of the active component, thecontribution of said electrode to the EMF of the battery at 25° C.varies continuously with the state of charge by more than 0.05 V, forany amount of Li present in said material.